"UNHCR is alarmed by reports we're receiving from our field offices of a dramatic rise in the frequency and brutality of the Lord's Resistance Army's attacks against civilians in the DRC and Sudan in the last four months, and increasingly in the CAR," said Melissa Fleming, spokeswoman for the UN High Commissioner for Refugees.

"It just seems to us that this awful, murderous group has only one agenda and that is to terrorise, rape and kill with unimaginable brutality," she added.

The LRA went on at least 10 rampages in the Central African Republic's southern Haut-Mbomou province between March 20 and May 6, killing 36 people, burning houses and sending 10,000 people fleeing.

They killed up to 100 people in their last major attack in the DR Congo between February 22 and 26, according to reports collated by the UNHCR.

Civilians in Sudan, particularly in regions bordering DR Congo, Uganda and the Central African Republic, also suffered attacks by the Ugandan rebels.

At a refugee settlement in Sudan's Western Equatoria, rebels killed one man and injured another, before being repulsed by South Sudanese police.

The LRA took up arms in 1988 in northern Uganda and has acquired a reputation for brutality. Its leaders are wanted by the International Criminal Court for war crimes.

Since 2005, under pressure from the Ugandan army, the fighters pulled back from their bases in Uganda to move into the remote northeast of DR Congo, where they were said to number fewer than 100 late last year, according to the UN mission in the country.

"There are claims that the size of the Lord's Resistance Army has been reduced, but it is clear that their killing power has not subsequently gone down because what we're seeing here is a rise in brutality," said Fleming.

According to the UNHCR, the LRA has killed over 1,800 people in DR Congo since December 2008. Some 280,000 people have also been forced to flee their homes after coming under attacks.

In Sudan, the rebels have killed up to 2,500 people and displaced 87,800, said the UNHCR.